1. Field of the Invention
The present invention pertains to an impedance measuring apparatus and in particular, relates to an impedance measuring apparatus with which high-speed measurement is possible.
2. Background of the Art
Impedance measuring apparatuses that operate by the automatic balanced-bridge method are an example of the prior art of impedance measuring apparatuses. Impedance measuring apparatuses that operate by the automatic balanced-bridge method are characterized in that they cover a broad measurement frequency range and their measurement accuracy is good within a broad impedance measurement range.
The internal structure and operation of an impedance apparatus that operates by the automatic balanced-bridge method are described below. FIG. 1 is a drawing showing the internal structure of an impedance measuring apparatus that operates by the automatic balanced-bridge method. Impedance measuring apparatus 10 in FIG. 1 comprises signal source 200, current-to-voltage converting apparatus 300, and vector voltmeter 400 for determining the impedance of device under test 100. The entire impedance measuring apparatus 10 is operated under the control of an operation control device CTRL1 (not illustrated), such as a CPU.
Device under test 100 is an element or a circuit having two terminals. Device under test 100 should have at least two terminals and also, it can be an element or a circuit with three or more terminals. In this case, two of the three or more terminals are used for the measurements. Device under test 100 is represented by “DUT” in FIG. 1. The point where device under test 100, cable 510, and cable 520 are connected in FIG. 1 is referred to as the High terminal. Moreover, the point where device under test 100, cable 530, and cable 540 are connected is referred to as the Low terminal.
Signal source 200 is the signal source that is connected to a first terminal of the device under test 100 by cable 510 and generates measurement signals that are applied to device under test 100. Moreover, signal source 200 is also connected to vector voltmeter 400 by cable 510, cable 520, and buffer 550 and feeds measurement signals to vector voltmeter 400. The measurement signals are single sine-wave signals. However, the measurement signals are not limited to single sine-wave signals and can also be signals that comprise several sine waves.
Current-to-voltage converting apparatus 300 converts the current that flows to device under test 100 and outputs voltage signals to buffer 560. Current-to-voltage converting apparatus 300 comprises a null detector 310, a narrow-band amplifier 600, a buffer 320, and a range resistor 330. Cable 530, null detector 310, narrow-band amplifier 600, buffer 320, range resistor 330, and cable 540 form a negative feedback loop 340.
Null detector 310 balances the current that flows to range resistor 330 and the current that flows to device under test 100 and outputs signals to narrow-band amplifier 600 such that the current that flows into the input terminals of null detector 310 through cable 530 will be brought to zero. When the current that flows to range resistor 330 and the current that flows to device under test 100 are balanced, the current at the Low terminal is kept at virtual ground.
FIG. 2 is a drawing showing the internal structure of narrow-band amplifier 600. Narrow-band amplifier 600 comprises a phase sensitive detector 610, a filter 620 and a filter 630, as well as a vector modulator 640, and amplifiers and amplifies the output signals of null detector 310 and outputs them to buffer 320. Narrow-band amplifier 600 resolves the output signals of null detector 310 into an in-phase component and an quadrature-phase component using phase sensitive detector 610, filters the in-phase component and quadrature-phase component using filter 620 and filter 630, modulates the filtered in-phase component and quadrature-phase component using vector modulator 640, and feeds the vector-modulated voltage signals to buffer 320.
Phase sensitive detector 610 is a quadrature detector and comprises a mixer 611, a mixer 612, a signal source 613, and a signal source 614. Signal source 613 generates sine-wave signals and feeds them to mixer 611. Moreover, signal source 614 generates cosine-wave signals and feeds them to mixer 612. The sine-wave signals output by signal source 613 and the cosine signals output by signal source 614 have the same frequency as the measurement signals and they are orthogonal to each other. Consequently, mixer 611 and mixer 612 can orthogonally resolve the output signal of null detector 310 into an in-phase component and an quadrature-phase component.
Filter 620 is an integrator that comprises a resistor 621, an amplifier 622, and a capacitor 623, and integrates the output signals of mixer 611. Filter 630 is an integrator comprising a resistor 631, an amplifier 632, and a capacitor 633, and integrates the output signals of mixer 612.
Vector modulator 640 comprises a mixer 641, a mixer 642, a signal source 643, a signal source 644, and an adder 645. Signal source 643 generates sine-wave signals and feeds them to mixer 641. Moreover, signal source 644 generates cosine signals and feeds them to mixer 642. The sine-wave signals output by signal source 643 and the cosine-wave signals output by signal source 644 have the same frequency as the measurement signals, and they are orthogonal to each other. Mixer 641 modulates the sine-wave signals that are output from signal source 643 with the output signals of filter 620 and outputs the modulated sine signal. Mixer 642 modulates the cosine-wave signals output from signal source 644 with the output signals of filter 630 and outputs the modulated cosine signal. The voltage signals output from mixer 641 and the voltage signals output from mixer 642 are added by adder 645 and output to buffer 320.
Vector voltmeter 400 of FIG. 1 measures output signal Edut of buffer 550 and output signal Err of buffer 560. Control device CTRL1 calculates the vector ratio of signal Edut and signal Err that have been measured and calculates the impedance of device under test 100 from the calculated vector ratio and the resistance of range resistor 330.
Measurement of the gate oxide film is one important measurement in the production of MOS devices. The gate oxide film thickness is an important parameter in determining the operating threshold of MOS-type devices. The gate oxide film thickness is measured by measuring the impedance of an MOS device, calculating the capacitance from the impedance measurement, and converting this calculated capacitance to the equivalent oxide film thickness using the dielectric constant.
When an MOS device on a semiconductor wafer is tested using a conventional impedance measuring apparatus 10, a wafer interface device comprising a switch matrix, a chuck, a probe card, and the like is added between the impedance measuring apparatus 10 and device under test 100. The wafer interface device has a larger ground capacitance than device under test 100. Moreover, cable 510, cable 520, cable 530, and cable 540 that are connected between this wafer interface device and impedance measuring apparatus 10 are relatively long and also have a large ground capacitance. Cable 510, cable 520, cable 530, and cable 540 are called cable 510, etc., hereafter. FIG. 3 is a drawing in which the above-mentioned ground capacitance has been added to FIG. 1. Ccable in FIG. 3 is the total ground capacitance of cable 510, etc. Moreover, Cwinf is the ground capacitance of the wafer interface device. The ground capacitance of the wafer interface device comprises the ground capacitance of the switch matrix, the ground capacitance of the chuck, and the ground capacitance of the probe card.
Conventional impedance measuring apparatus has two problems with high-speed measurements. The first problem is that when a large ground capacitance is applied to the Low terminal, the current-to-voltage converting apparatus 300 takes a long time to settle. If the time to settling of the current-to-voltage converting apparatus 300 is long, the time until the current that flows to range resistor 330 and the current that flows to the device under test are balanced is also long and the wait time until measurements begin is increased. When the capacitance of an MOS device on a semiconductor wafer is measured, this problem is exacerbated by a wafer interface device and cable 510, etc., with a large ground capacitance, as described above.
The second problem is that when the capacitance of an MOS device on a semiconductor wafer is measured, the ground capacitance of the wafer interface device and cable 510, etc. is not constant. There are many types of wafer interface devices and cable 510, etc. depending on the device under test and the user's selection. Consequently, the ground capacitance of the wafer interface device and cable 510, etc. is not constant. Unless the ground capacitance on the wafer interface device and cable 510, etc. is constant, it will be very difficult to keep the ground capacitance from affecting the measurement results as planned.
There has been considerable progress in microfabrication technology for semiconductors in recent years, with a huge number of elements or circuits being formed on one wafer. While there has been an obvious increase in the number of elements that serve as the device under test, a corresponding increase in measurement time is not allowed. Moreover, sacrifice of measurement precision for high-speed measurement is not acceptable. The realization of high-speed, high-precision impedance measurement is a very important problem in the semiconductor industry today.